text:
How does this vortex form inside a jet engine?
I have seen this in a picture once online and thought it was very strange and wondered about the formation of this vortex and how could it happen.

Source
Not everyday do you see this happening. So how does this happen, and why does it not happen very often?

Source
Another picture of this strange occurrence. This generally happens with thrust reversal.

why does it not happen very often. Those vortices are always there.  It's just that you only see them when they pick something up like dust or water.  They are just a function of the engine sucking in large amounts of air.

Anyone else find those vortices look like the time bubbles from Donnie Darko?

I'd be even more interested in why the YC14 inlet vortex is going in the top of the engine inlet, while the vortex on the C17 engine is slightly below center.

Whenever there is under-pressure in air, the air flows along the gradient of this under-pressure, i.e. into the direction of the source of the under-pressure. (In first order.)
If there's a wall (or the ground) in the way, air will flow along the surface of this obstacle and will change its direction at some point, as shown in this sketch:

Now, imagine the whole air around the engine is rotating slowly for some reason around the symmetry axis in the sketch.
From physics, you may know the conservation of momentum, or in this case, of angular momentum. It says that the product of distance to rotation axis times RPM of this rotation is constant for a certain particle. If a particle reduces its distance to the axis, its RPM increases.
This is like the spinning ice skater, who rotates faster when he pulls his arms to his body (to the axis).
This law also applies to packets of air, which are sucked to the axis, though internal friction brakes the air to some extent.
The fast rotating air near the axis encounters a centrifugal force, i.e. the air is pulled away from the axis, forming a region of strong under-pressure. In this region, the air expands and so, cools down. This can cause condensation of water, if the humidity of the air is high enough. And that's what you can see on your pictures.
There's still one question unanswered: I wrote, this happens if the surrounding air is rotating slowly. This usually happens, because the wind is deflected from the aircraft itself, which means it changes its direction, which is some kind of rotation.
And of course, a strong, non-uniform air flow from the rear, as generated by the reverse thrusters, can cause more / stronger rotating air, which results into stronger vertices. So, it's no wonder when they appear (or better: become more visible) in this case.
Here is a nice picture of a simulation, which also shows that the fuselage can be the wall where the vortex is created.

Source: http://gallery.ensight.com
It seems the wind comes from the camera position, or a bit more right of it. It gets deflected by the fuselage and flows along it, which explains the straight lines coming from the rear and the curved ones coming from the front left of the aircraft. If you imagine the air flow along / under the fuselage, you may also understand why the two vertices are rotating in different orientations.
As the strong under-pressure inside the vortex, it may also suck in dust or even larger objects, which can damage the engine. For aircraft landing on unpaved airfields, unpaved strip kits may be available, which can contain a vortex dissipator:

Source: http://www.b737.org.uk
This is a nozzle blowing bleed air into the region where vortices could apper, reducing the under-pressure and so the risk of ingesting objects from the ground.

Notes:
You can see something similar in the sink of your bathtub. As soon as there is a little rotation, it is amplified above the sink, the centrifugal force pushes the water aside, and forms this fast rotating vortex.
A tornado is a giant version of an inlet vortex. The air is moving along ground over far distances, until it moves upwards at some point, and a slight rotation leads to this fast rotating hose sticking to the ground and sucking in everything due to the strong under-pressure inside.
And a bit off-topic:
Sometimes, you can see vortices at the wing tips or at the edges of engaged flaps of aircraft flying through humid air. The reason is similar: The air is put into rotation due to the higher pressure below and the lower above the wings. (But that's not exactly the same as inlet vortices.) And sometimes, you also see fog forming above the wing just because of the under-pressure.
On this picture, you see both, and may imagine the rotation for the second vortex from the right.

Source: http://avioners.net

That picture of the vortex sucking at the fuselage reminds me of a Vulcan accident.  High up on the port side of the nose was a compartment containing a fire axe and some fire proof gloves.  The compartment hatch had not been correctly closed and when the engines ran up for take off, the hatch popped open and the engine sucked out one of the gloves and slurped it up with a big bang, an impressive flame and lots of smoke.

Does this happen in the middle of the flight like at cruising altitude of about 40,00ft?

@JanHudec shouldn't this vortex dissipate when air is hitting it at 500mph.

@Ethan, correcting myself. No, it doesn't. At high speed, the engine actually captures air from less than its diameter, so there is no flow towards the centre and consequently no acceleration due to moment of inertia.

The inlet vortices shown form in the case of turbine engines and in propellers during operation near ground where there is a little or no crosswind. In standard atmospheric conditions, the vortex is invisible; however, with enough humidity, condensation occurs inside the vortex core (as temperature falls below the dew point temperature), and the vortex becomes invisible.

Source: wpi.edu
As the engine is started, the engine ingests air whose cross sectional area is greater than the engine frontal area. Near ground, this includes ground as well and induces a flow there. As air nears the engine, it is accelerated and if some amount of vorticity is already present, it forms an inlet vortex, which become visible under some conditions.

Image from patent application of Active ground vortex suppression system for an aircraft engine EP 1413721 B1
The inlet vortex forms between the engine (or propeller) and a stagnation point on the ground. This is usually not formed far away from the ground as the vortex lines cannot start or end in a fluid according to Helmholtz's theorems.
As a result, one end of the vortex is attached to the engine (or propeller) and the other end extends and attaches itself to the ground.
Two mechanisms for this inlet formation have been proposed,

Ambient vorticity (upstream) is amplified by the engine and stretched and drawn into the engine.
Without upstream vorticity (i.e. in an irrotational flow), the vortices form in the presence of crosswind. In case of high crosswinds, the inlet vortex is bent around the inlet and for very high crosswinds, the vortex does not form at all.

The study of engine inlet vortex formation is an important research topic as the inlet vortex can cause a number of problems such as,

FOD ingestion
Compressor surge
If there is a crosswind, the inlet vortex can cause separation in the inlet lip region, causing performance losses.

For the inlet vortices to form, there should be usually some sort of rotation flow in the far field, which is usually given by the crosswind as it interacts with the fuselage; In case there is no crosswind, inlet vortices still form, though a pair are formed, which rotate in opposite directions. This is probably the vortices in simulation which @sweber showed in answer..

Source: gallery.ensight.com
One reason for the presence of vortices when reverse thrusters are engaged is that they cause air flow with sufficient vorticity in the vicinity.
See An Experimental Study of the Formation and Unsteady Characteristics of Inlet Vortices by Wang and Gursul, accessed via enu.kz
Note: The vortex suppression system in the patent described above injects fluid near the inlet, which disrupts the flow structure of a ground vortex, thus mitigating ground vortex.

Image from patent application of Active ground vortex suppression system for an aircraft engine EP 1413721 B1

Sometimes it's not water being condensed out of the air, but rather water being picked up off the ground (e.g. during or shortly after rainfall.) It's kind of fun to watch if you're sitting around on a wet taxiway waiting for clearance.

I'm not convinced that it's condenstation at all.  I've spent many hours around powerful jets and have only ever seen these vortices when it's wet or dusty.


text:
Why don't airliners have rudders on the winglets?
There are a couple airplanes out there that use the winglet as the rudder, but why doesn't the winglet have a rudder on it to increase the turn of the aircraft?

why can't, again. Will you learn to ask why don't and stop making false assumptions?

@Federico perhaps you could phrase it more kindly ? Please bear in mind that users might be young or non native English speakers. This kind of difference can be quite subtle and not easy to get right for everyone.

@Antzi this is most definitely not the first (nor second or third) time this gets mentioned to him. One can allow for age and all other kinds of issues, but if advice keeps falling on deaf ears kindness and empathy start to wear off.

The purpose of the rudder is to control the heading of the aircraft. It turns the aircraft by creating an aerodynamic moment about the vertical axis of the aircraft. The moment is product of the aerodynamic force and the arm.

image: NASA
The aerodynamic force is the the side force on the rudder, the arm is the distance between the rudder and the centre of gravity of the aircraft.
If the rudder would be on the winglets, the side force of the rudder would act at a short distance behind the centre of gravity, reducing the effectiveness very much.
It would be possible the have airbrakes on the winglets / wingtips and use differential braking to create the rotating moment. This is done for example on the B2 which doesn't have a vertical stabilizer. The added complexity and weight increase makes it an unattractive option for conventional aircraft.

Note that the Horten flying wings had small airbrakes close to the wingtip for yaw control. They were OK at high speed, but had so little effect at low speed that the second prototype of the Horten IX was lost when one engine quit.

Of course, if the wing is either very far back compared to on a typical aircraft (such as with the Long-EZ in the OP's photo) or is very long and highly-swept, then the winglets can easily be far enough behind the COM to be useful vertical stabilisers/rudder mounts.

Short answer:

Vertical tails create more yawing moment per unit of drag.
Vertical tails create less adverse rolling moment for a given yawing moment.

The rudder works by creating a side force, just like a wing creates lift. This incurs a small amount of drag (about a tenth of the side force).
Even on a swept wing their lever arm in lengthwise direction is small compared to the wingspan. If the winglets would be used as rudders, they would be most efficient as drag devices, because then their force will act on a longer lever. This has been done on flying wings (note that the SB-13 tailless glider has a maximum rudder deflection of 70° in order to create drag).

SB-13 in flight (picture source)
This makes the use of a rudder vastly more efficient for creating a yawing moment than using winglets.
The deflection of winglet rudders will also change the lift distribution on the wing, so they do not only create a yawing moment, but also a rolling moment. Vertical tails wich are high above the roll axis also add a rolling moment, but this is comparatively small to the rolling moment of a winglet with rudder on a high aspect ratio wing. In both cases, the rolling moment acts against the desired roll direction, so it is preferable again to create the yawing moment with the vertical tail.

Couldn't you use both winglet-rudder at the same time to prevent the rolling moment?

@ROIMaison: If both are behind the center of gravity and try to contribute to the desired yawing moment, their side force has to point away from the center of the desired circle. Thus, the outer winglet will induce a downforce on the wingtip, and the inner one additional lift on the opposite wingtip (if the winglets point up). Both will produce their own adverse rolling moment.

Ah yes, I overlooked this fact. Thanks for the explanation

It is possible, but just not that useful.
Controllability
In order to maximize the moment incuded by the rudder (or keep the rudder as small as possible to provide this moment), you want to place the rudder as far (in longitudinal sense) from the center of gravity. This increases the moment induced, and thus the effectiveness of the rudder. In a normal airliner, you'll have a tube with wings in the middle, and thus the most aft part to place the rudder is at the end of the tube. On the aircraft in your picture, the most aft part is the wing tip, so it makes sense to put the rudder there.
Structural reasons
If you place the rudder at the end, this will mean there will be substantial forces induced at the tip of the wing if you use the rudder. This means that you will have a substantial bending moment added to the wing, making it much heavier.
This is further discussed in Winglet vs span extension

Nice answer. Perhaps you could add why the rudder is generally not at the front of the aircraft ?

I could, but it's not related to the question, is it?

Strictly, it isn't, unless you consider the broader question of positioning the rudder

@Antzi: Look here for the answer.

The existing answers miss what seems to be a fairly obvious answer: the winglets are an afterthought, added to existing, proven designs (e.g. 737) to improve fuel economy.  Putting rudders on them would require running control cables, perhaps strengthening the wing structure, &c, rather than just bolting on new tips.
You could perhaps design a new airliner from scratch to do this, but again, why abandon a proven basic design unless there are large gains to be had?

For example, the 787 was a brand new design, which had winglets right from the start. So all you have to say is, "Well, if they were any good, the 787 would have them." OK, so we conclude that they're probably not any good, but the question is why.

@David Richerby: For a brand new design, it looks quite suspiciously similar to the 757, 767, & 777, not to mention products from Airbus and others.  Basically another iteration of the standard tube with wings and a tail, and a couple of engines hanging below the wings.  When I said "new design", I meant perhaps something like the X48 and similar blended wing-body designs.

Now you're just contradicting yourself. If the 787 doesn't have rudders on winglets because it's a tube with wings, I put it to you that the 737 not having rudders on its winglets has nothing to do with the difficulty of retrofitting them to an existing design, which is the only reason you give in your answer. The 737 is also a tube with wings, you see.

@David Richerby: I don't see the contradiction.  We (well, aeronautical engineers :-)) know how to build "tubes with wings", so each new iteration improves on that.  E.g. the 787 reworks the basic design using composites (yes, I'm simplifying).  But to put rudders on winglets, you have run control lines out to the tips, strengthen the outboard parts of the wings to handle the different stresses, decide what to do with the elevator now that you don't have a vertical tail... Which also applies to retrofits: why add rudders out there when you already have a perfectly functional tail?

You're claiming that the 737 and 787 are just tubes with wings, and therefore the same considerations apply to them. But the 737 has no rudders on its winglets for one reason (hard to retrofit to an existing design), while the 787 has no rudders on its winglets for some completely different reason (though you haven't clearly stated what). That is a contradiction: the 737 and 787 cannot simultaneously be the same and different.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Does this type of stabilizer exist?
If they do then what aircraft are they installed in? This stabilizer is not a T-tail shaped stabilizer.

As you can see there are 2 v-shaped vertical stabilizers with a horizontal stabilizer hanging on the each of the vertical stabilizers. So different from a V tailed. And what advantages would this design have over a t-tail for example?

Are you asking about a v-tail, or is your model different?

@Pondlife Different look at the 2 vertical stabilzers and you will see that each stabilizer has a horizontal stabilizer hanging off the end of it.

@mins I did a huge amount of research trying to find this type of tail online and couldn't find that's why I asked this question just to see if somebody knew about this design because I want to learn more about it and incorporate it on my airplane design. I think maybe I should draw it on paper, its kind of hard to tell what I'm asking about.

Paper airplanes and real ones can't be compared easily, they work at different aerodynamic scales with different Reynolds numbers (though new flying devices come close and even further in downscaling), and mostly in turbulent air. If you can't find an implementation of this empennage mixing V and T tails, then it's likley because more optimized ones exist for actual airplanes. If you're interested in knowing why, then why not move the question in this direction?

Doesn't seem OT to me - it is about aircraft. It might make some scratch their heads and ask "why", but the kid's showing some outside the box thinking, and that's what got man flying in the first place.

It looks like a variation of a Pelikan, which has never actually been flown.

I don't know if such a structure actually exists, but I find it difficult to see why anybody would build one.
Firstly, placing the horizontal stabilisers at the top of two separate tail fins will require that each fin is strengthened to support the extra weight and the aerodynamic loads,
Secondly, there will probably be any number of aerodynamic issues around the joints between the components, each of which will be doubled.
Thirdly, in a real aeroplane you will have introduced substantial complexity into the control systems that have to manage all of this.
There may be other issues, but I see very little benefit aerodynamically or structurally to such a layout.
A conventional V-tail involves some control complexity, but does away with the weight of the vertical stabiliser. A T-tail retains the vertical stabiliser, but avoids control complexity, aerodynamic issues that may occur close to the engines, and can use the extra length to improve the leverage available to the horizontal stabiliser.
All that said, aircraft design is about compromise and utility. There may exist some powerful reason for adopting a sub-optimal layout to fulfil some specific purpose. You can see such an example in the design of the AN-225 Mriya which was originally built to transport the Soviet Buran spacecraft. The vertical stabilisers were moved to the ends of the horizontal stabilisers to keep the clear of the wake from the spacecraft riding piggy back. The 747 used to transport the US Space Shuttle had a similar modification, but retained the original vertical stabiliser.
Soviet Buran spacecraft during transport:

US Space Shuttle Atlantis during transport (image source: Wikipedia/NASA):

I wouldn't say that it has very little benefit because I put this type of tail on a paper airplane and it flew just fine and wasn't difficult to build it either.

Interesting paper on tail design, by Mohammad Sadraey, Daniel Webster College. Includes rationale to select a design over the other.

I'm not sure the application of anything to a paper airplane is going to show benefits or downsides. There's just not enough in common with an actual aircraft.

I'd also add that for a full size plane this design would lead to relatively high bending stresses at the top of the fin, due to the abrupt change from horizontal to near vertical. This would be worse when the tail is generating a high pitching moment.  (I agree with all points in the answer by the way, this is just another bit of detail for Ethan to consider looking into.)

As far as I know, and my searching for weird/unusual/concept aircraft has yet to reveal this type of design in use or proposed... even if it appears familiar or should have been proposed. The kicker here is the V-component. You can get tandem rudders, in various configurations, inverted V, V, split Vs, or V and elevators. The design here is V, split, with a T-top
[Hopefully someone that can explain the aerodynamics better will come along] Generally, this type of design would be considered redundant and too much extra weight for no general gain. A V type configuration will generally replace both the Rudder and Elevator into a single component. To then re-add an horizontal stabilizer would be redundant. Additionally you'd have to have more support structure in the V to be able to support the horizontal stabilizer and the forces it'd experience.
From the image, it looks like a paper airplane, and if that type of design works, it's probably more due to the addition of more lifting surfaces that give aid towards better stability.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Does an accelerating airplane also start climbing?
Assume that an airplane is flying level. If I understood correctly in this situations there are four forces acting on it: weight, thrust, drag and lift.
Lift depends on air density, airspeed, wing area and lift coefficient. In the situation described above all of these parameters are constant with the exception of airspeed which can be changed by pilot. So when increasing airspeed lift increases too which makes sense because that seems to be how planes take off.
However when you watch footage of airplanes accelerating (for instance when a fighter turns the afterburners on) they seem to be able maintain altitude.
At this point I see three possibilities:

The airplane doesn't climb or climbs very little because the pilot simultaneously compensates.
I have the wrong impression and the airplane does climb.
My reasoning above was wrong and there's actually no reason for the airplane to climb further.

So which one is it?

Many pilots are taught by the concept that Throttle equals Height and Elevators equal Speed, as you've just discovered. Obviously it's all interlinked, but the way most people think of an aircraft climbing and descending with the elevator as an up/down control is wrong.

Possible duplicate of Trim setting and constant airspeed?

See also pitch vs. power.

@Dan - I was taught that throttle equals energy, elevator equals AOA. The amount of energy you're adding into the equations for the four forces is directly related to throttle, and your lift and drag coefficients are directly related to your AOA. This is a mathy way to approach it which isn't for everyone, but I'd had experience with PC flight simulators  growing up, and so was very familiar with the basic behaviors of stick and throttle and some of the fundamental physics of flight (like trading speed for altitude and vice versa, up to and including energy-management ACM theory).

What about this aircraft ? (I mean, induced AOA changes due to increasing thrust. Marginal, as the aircraft will pitch up soon enough, but yet, basically it does the opposite in the first few seconds, right ?)

You assume that three of the four forces are constant. This is not quite right: Only the mass is given, all the other forces are variable.

Thrust $T$: Changes with throttle setting. Also, thrust can vary with flight speed $v$ in a way that can be expressed as $T \propto v^{n_v}$.
Lift $L$: Depends on speed and on angle of attack $\alpha$, as in $L \propto v^2 \cdot (\alpha - \alpha_0)$. The accelerating airplane is held level by gradually reducing the angle of attack, such that lift exactly equals weight. The elevator is the instrument for controlling angle of attack.
Drag $D$: Like lift, drag depends also on speed and angle of attack: $D \propto v^2\cdot (\alpha - \alpha_0)^2$

Your option a is the correct one: The pilot compensates by adjusting the angle of attack by gradually pushing the stick forward. If he would keep the stick (and consequently the elevator) unchanged, the airplane would climb.
Now it might be opportune to introduce the concept of specific excess power ($SEP$). This is the extra power available to the pilot, related to the aircraft weight to make it comparable between airplanes. $$SEP = \frac{P_{eff}-D\cdot v}{m\cdot g}$$
where $P_{eff}$ is the effective power: $P_{eff} = T\cdot v$. When the pilot commands more thrust than needed for level flight, the excess thrust can be either converted into kinetic energy (speed) or potential energy (altitude). If you look at the units, $SEP$ is a speed. It tuns out that it is the possible climb speed available if flight speed stays constant: $$SEP = v_z$$
In reality, the practical climb speed is smaller because the airplane needs to speed up when climbing to compensate for the decreasing air density.
The possible acceleration is almost as easy to calculate from the specific excess power: $$\ddot x = SEP\cdot\frac{g}{v}$$
Of course, a mixture of both is also possible, as is trading in one for the other. By adjusting the elevator and with it the flight path, the pilot decides what happens.

Your (a) assumption is correct -- the aircraft can accelerate without climbing because the pilot compensates.  As the speed increases, an aircraft that was stable & in trim would naturally tend to climb to maintain airspeed, if the pilot didn't intervene.  To counter this tendency, the pilot applies a slight nose-down input (i.e. elevator down, raises the tail & lowers the nose -- control stick forward).

And if the plane uses a fly-by-wire system, this same pitch compensation can be applied by the flight director computer. Most Airbus airliners in service have such a system which compensates for this basic tendency, allowing a more "ideal" use of the sidestick and throttle by the pilots (though, after 1500 hours of flight time in aircraft with less sophisticated control setups, there's some unlearning to do when you sit in an Airbus for the first time). The latest Boeing widebodies and 747-8 have a FBW, but it's designed to be more similar to a pure mechanical system and doesn't compensate.

A basic stability requirement for (most) a/c requires the plane to pitch down when speed decreases and pitch up when speed increases. It's called positive speed stability.
As you decelerate the nose of the plane will point down and the plane will pick up some speed as it begins to descend. It will do so until it picks up a little too much speed and the nose now starts to point a little up, in doing so slowing down a little bit and so on. It's called a phugoid oscillation.
The very same thing will happen when you floor the throttles: speed increases, airplane wants to point the nose towards the sky, plane slows down a little bit, that makes the aircraft want to point the nose a bit below. It wobbles a little bit and then it settles in a nice gentle climb at the very same speed you had before you have pushed the throttle.
It is not the increased lift that makes the a/c climb(in fact you will have less lift as you are climbing and a reduced AoA), it is the stability momentum that pitches the nose up and that makes that extra power from the engines point up. That is what makes you climb.
So how do pilots maintain level flight while accelerating? Pushing the control forward will instruct the tailplane to create less downlift, raising the tail, lowering the nose and counterbalancing the nose up intentions of the plane. You are now still flying level, a bit faster, and a bit more "nose down" mind you: because of the extra speed you now have a lower AoA than before, so your nose is a few minute degrees lower than before when you were slower.


text:
Why do eastbound transpacific flights fly at lower latitudes than their westbound counterparts?
I’ve seen some routes for flights from Asia (mainly Hong Kong and Shanghai) to the west coast of North America, which fly at lower latitudes than flights in the other direction (which they fly closer to the North Pole). From what I know, the jet streams go eastward, and are stronger near the poles. So why don't flights from Asia to western North America fly closer to the pole to take advantage of stronger tailwinds? Or did I get it wrong?

The jet streams don't occupy fixed positions.  You'd either have to find actual winds aloft for the date of those flights, or do a statistical analysis of flight paths over a fairly long period.  Here's a link to a real-time map of jet streams: https://www.netweather.tv/charts-and-data/global-jetstream#2019/12/14/1800Z/jetstream/surface/level/overlay=jetstream/orthographic=-169.19,51.02,712  Looks like today they're quite a bit to the south over the Pacific.

Also note there is a Pacific Track system similar to the North Atlantic Tracks, that is shifted north and south based on winds like the NAT.

The different routes are due to the enroute winds created by the jet stream. The following map shows the current winds over the Pacific:

(map from skyvector.com)
You can see strong winds from the West in the central Pacific region of up to 150 knots (see here for a legend on how to read wind barbs). A flight from Hong Kong to Los Angeles benefits from these winds. A flight in the opposite direction would experience this as a headwind though and will therefore try to avoid the region. You can see that the winds are much weaker in the North.
In order to organize traffic in the Pacific, a track system has been established: Pacific Organised Track System (PACOTS).

The Pacific Organised Track System (PACOTS) is a flexible set of aircraft route tracks that primarily link commercial air transport gateways of Japan and Southeast Asia and the gateways of Hawaii and the North American West Coast, as well as those in Texas.

(skybrary.aero)
The following map shows the current PACOTS tracks:

(map from skyvector.com)
The green tracks are for westbound flights and the blue ones for eastbound flights. Due to the changes in the jet stream, these tracks are re-defined twice a day:

The tracks are developed twice daily by the Japanese Air Traffic Management Centre (ATMC) in coordination with Oakland ARTCC. Eastbound tracks are valid from between 0500Z and 1000Z (depending on origin) to 2100Z (with specified crossing time windows for 160E) and are given a numeric designator. The opposite direction westbound tracks are valid from 1900Z to 0800Z (with specified crossing time windows for 160E) and are designated alphabetically.

(skybrary.aero)

For emphasis/clarity: "the current PACOTS tracks" were for the 12 hour period when this answer was written, since they can change two times per day.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How do aircraft discern that an ACARS message is being sent to them?
This may be a simple question, but how do airplanes know which ACARS messages are meant for them from the ground? I'm not sure how physically close planes would have to be in order for that to be an issue. Do they simply receive all ACARS transmissions in the area and the CMU checks for this information in the header and continues to route it if it has the proper header? I'm assuming the VHF doesn't perform any checks since the data is still an RF signal at that point.

Every ACARS message includes the tail number of the aircraft it is to/from. Every aircraft decodes every message that it receives and then discards any that don’t match its own tail number. Since every tail number is unique, this is mostly sufficient.
The messages also contain a flight number, if applicable, which helps to distinguish a message about the current flight vs the last/next flight that aircraft may also operate that day.

A typical ACARS VHF transmission.
Mode A
Aircraft B-18722
Ack NAK
Block id 2
Flight CI5118 <<< looks like an identifier the receiver can filter on
Label B9
Msg No. L05A
Message /KLAX.TI2/024KLAXA91A1
https://en.wikipedia.org/wiki/ACARS


text:
Is Rutan's method of foam core construction obsolete?
Is Rutan's method of foam core construction obsolete, or is it used on other fibreglass designs as well?

Still used when appropriate at Scaled Composites. It’s more associated with home built aircraft since molds aren’t required.

I was a regular at Oshkosh during Rutan's heyday in the 70s, and had a chum that was building a Dragonfly, which used the same construction method.  Rutan's mold-less process is only suitable to one-off projects, like prototype parts, or amateur built airplanes.  It's a sub-optimal process from an engineering perspective because it is very time consuming to produce an ascetically decent component, and tends to be somewhat heavier than molded sandwich parts.
Rutan advertised extreme speed at creating a structure compared to metal or wood/fabric.  It was much faster to create a wing or fuselage... but then all the time saving was eaten up in finishing.  The method leaves a rough and somewhat uneven outer surface (due to the hot wire cutting process, where the wire bends under stress as it's cutting, producing a dished result when cutting a curved profile) that requires hours, and hours, and hours of filling and sanding (using looooong sanding blocks) with a lightweight filler made from epoxy and micro-spheres (hollow glass beads) to get anything close to the finish you get with a mold.
The hot wired foam cores, done by hand, always have surface continuity imperfections that need lots of filler, plus the outer surface of the epoxy tends to be rough.  You have to sand very carefully to avoid going down into the glass cloth.  The original layup method didn't use vacuum bagging and produced a heavy part with excess epoxy resin.  Later on builders started to vacuum bag the layups and use peel-ply fabric (polyester fabric similar to aircraft covering fabric, which epoxy won't stick to) with an absorbent material to soak up excess epoxy and provide a better outer surface (which ended up with about the smoothness of the peel-ply fabric, the outer surface being an impression of the peel-ply weave.
My chum sold the Dragonfly project before finishing it.  He just didn't have the time to do all the finishing work.
If you are going to build your own airplane from glass and foam, that method is pretty much the only option.  Beside Rutan's designs, the EZ's and Quickies, and the Viking Dragonfly, I can't think of any other designs off hand.  There was another scratch built homebuilt that used sheets of polyurethane foam bent to the skin curvature on a form, then layups done on the skin, that produced something closer to a modern fibreglass sandwich system without using molds, but I can't recall the name.
Here's is a complete guide to the Rutan moldless method.


text:
Why do some GA aircraft use bungees or torque tubes vs. cables for control surface actuation?
Why do some GA aircraft use bungees or torque tubes vs cables for control surface actuation?
I understand using cables is the most common for GA aircraft.
I understand bungees are common in gliders due to low control surface forces, the bungees being in tension and adding more control surface feedback to the pilot.
I understand torque tubes are used on faster planes. Eg. 300 mph+ to minimize control surface flutter.
Why would torque tubes be used on a "slower" plane like the Varieze?
Are these assumptions correct?

You might want to edit your title since you answer that question in your question body...

Cables are favoured when the routing between input and output is complex and long. However, if the runs are vary long, cable stretch starts to become a problem, which creates springy-ness in the control circuit, creating a rubbery feeling at the stick.
Push rods are used as an alternative when the runs are shorter. You can use push rods for longer runs by using idler bell cranks joining multiple push rods in a chain, or sliding support bushings at the push rod mid point that allows a longer push rod than an unsupported one.
Torque tubes are an alternative to push rods, but can be less rigid and can be similar to, or worse than, a cable circuit in having springy compliance (it's basically a long torsion bar, like some car suspension springs). Whereas a push rod system, if designed properly, the only compliance in the system is bearing backlash for rod ends and pivot points.
A torque tube's main attraction is packaging advantages; in something like an aileron control you can run the tube though a confined space along a wing without the need to allow for arcs of movement, at a bell crank for example. In the Varieze, a torque tube is the simplest and most compact way to control an aileron, via a small tunnel in the foam core of the wing that the torque tube operates in.
You wouldn't necessarily use a torque tube for flutter resistance in itself. A push rod aileron linkage would probably give a more rigid connection than a torque tube for the same total weight, depending on how long the assembly is. Compliance in the control circuit is a factor, but flutter resistance is mostly achieved by making sure the control surface itself has its center of gravity on or forward of the hinge line so that it can't return energy into the wing structure when the wing flexes by using mass balances (although this isn't always necessary if the structure is rigid, the speeds are low, and the control surface is light).
If the control surfaces are hydraulically driven, this doesn't apply and flutter resistance is achieved by the rigidity of the hydraulic actuator connection, often supplemented by hydraulic dampers between the structure and control surface that work like automotive shock absorbers.
A bungee is just a trimming device that tends to hold the control surface at a set point using springs that you adjust when you trim, basically applying pressure for you to set the hands-free neutral position of the stick. You could stretch a shock cord from the stick to the instrument panel and one from the stick to your seat, with the tensions set to hold the stick at position X unless you overpower them by moving the stick yourself.  Gliders use bungees for elevator trim because they do the job well enough, and eliminate the need and complexity of a tab at the control surface or an adjustable stabilizer (although some gliders, esp larger two place ones like the Blanik, do use elevator trim tabs instead of bungees).

Can you confirm my understanding that bungees are additional to control cables , not a replacement for or part of them?

When used as trim yes. They are used to apply a pull or push force, to hold the surface outside its natural trim point, so you don't have to. Linked in anywhere in the circuit. With hydraulic controls however, you might see a bungee strut installed between the cable circuit and the hydraulic actuator, so the cable  movement moves hydraulic input valve controls through the strut. In normal operation, the bungee acts as a rigid rod. If the hydraulic linkage jams, the bungee can be overpowered by the pilot so the other actuators can still operate. This has nothing to do with trim though.

Gliders use steel cables running in polyethylene tubes to have more friction in the linkage. Control forces are not an issue and the added friction helps to suppress flutter.

As I fly both an ASW 20 and a 24, and the 20 has more control friction compared to the 24, (because of the mixer mechanism?) which some pilots find objectionable but as a power pilot I find perfectly fine. The 20 has some quirks and maybe not the greatest ergonomics but has a more "solid" feeling and is effortless to maintain a pitch and roll attitude while thermalling. Some pilots don't like flying it. I love it.


text:
Can anyone identify this object?
In La Réunion, people are currently searching for plane debris that could be from the MH370 crash and this morning this object was found.

Can anyone identify it?

There is an unfortunately huge amount of junk just kind of floating around in the sea, and since this isn't obviously a large airplane part...it's hard to say if it's from MH370 or not.  It'll have to be take to a lab for analysis before anyone can really say anything.  (For more info on random junk in the sea: https://en.wikipedia.org/wiki/Great_Pacific_garbage_patch)

@JayCarr That's surely something to keep in mind ! Thank you.

its a toilet seat!

It's probably the remains of a Kenmore 325 sewing machine

@rbp From the space station?

Could be of of the overhead plastic panels which house the lights etc

We really shouldn't speculate, but it does not look like a panel piece for a door or window. Aside from two small warning lights on some models there shouldn't be very much.
This is very much speculation: Aside from the deep inset, which looks like it could have housed a CRT monitor, there are two holes for square aviation-style switches, and two holes, perhaps for backlighting, above it. The left hole looks very similar to an SD card slot.
Then again, it could have come from a boat or anything.

Really interesting, thank you !

DeltaLima is right. See It's a sewing machine


text:
Why does the bottom part of an aircraft nose curve upwards?

As you can see in this picture the bottom of the fuselage curves upwards to form the nose.

sorry if you guys do not understand my question please tell me what you think it means and I will tell you if that's what I mean.

If it didn't curve, how would you imagine it to be? Like a race car with unidirectional curve?

@Alexus It makes me think of my airplane design has a nose that doesn't curve up, but other than that I can't think of anything else. Can't show you a pic because I don't have it drawn yet.

@Alexus I mostly ask this question because of my airplane design nose to make sure its aerodynamic

What I am trying to understand is are you talking about slightly more predominant lower curvature over top curvature or just eliminating the whole curvature all together and having the tip of the nose aligned with the bottom line of the fuselage?

@Alexus Eliminating the whole curvature

@Alexus  Kind of like this /__

@Alexus Link Like this it has no curvature

Its hard to imagine fuselage that curves downward.

@Ethan, a curve like that car's would not be aerodynamically neutral; it would create a slight downward force on the nose of the airplane.

Having the tip of the nose in the centre gives best combination of aerodynamic performance, weight and internal volume. If you move the tip down, the slope above it will either be steeper, leading to higher drag, or longer, leading to greater length and thus weight; or you don't extend the plane, but then the longer sloped part will provide less internal volume. Only the cockpit windows, which need to be on the sloping part, shift the nose a bit down, to the centre of the cross-section just before cockpit.
Cars are different. For a car you want the bottom surface flat, because you don't want to push and compress air in the gap between the vehicle and the ground. But aircraft has air all around and it is most efficient for it to push air equally to all sides, so a circularly symmetrical shape works best.
Also note that lift is yet another thing. Lift requires a sharp, downward-sloped trailing edge.

A straight nose will have no pitch moment, while bending the centerline of the fuselage down in front will add a downforce there which will need to be balanced by more downforce at the tail.

The nose of the aircraft has to satisfy a number of requirements, some of them contradictory, like

The nose shape should produce the least drag
The pilots should have good visibility
It should house the weather radar and related avionics
The cockpit noise should be minimal

For example, the aerodynamic design of Concorde meant that the nose had to droop during takeoff/landing for good pilot visibility.
For aircrafts flying in transonic regime, some of the best nose shape from aerodynamic standpoint would be to have a symmetric body of evolution based on Haack series of curves.

"Nose Cone Haack Series" by Andersenman - Created with Gnuplot using the Haack series formula and a selection of input values for C relevant to the article. Values are 0, 0.333, 0.667, 1 and 1.333.. Licensed under CC0 via Commons.
However, due to fact that the cockpit is on the upper part of the nose region, this arrangement increases the complexity of cockpit windows (they will have to be curved in two axes, atleast) if the pilot has to have good visibility. As a result, center of the the nose is usually moved downwards.
This is the reason for the common profile seen in aircraft noses.
However, the disadvantage of this profile is that the pressure is high in the windshield region compared to the flat profile.

Source:AGARD LS-67, Paper-4
The main problem with (dual) curved cockpit windshield is the tighter quality control required and the associated costs. However, in 787, Boeing has achieved a continuous surface variation.
Source: airliners.net
It is to be noted that the aerodynamics is only one of the criteria for nose design. For example, a nose with similar profile to that of 787 was proposed for A350. However, it was later changed to accommodate other considerations.
Source: airliners.net
According to Didier Evrard, executive vice-president A350 programme,

There have been a number of trade-offs in the nose area, enabling us to maximise the volume of the cockpit and avionics bay while optimising aerodynamics and the positioning of the nose landing gear

"A350xwb nose 2009B" by Luis E. Contreras. Licensed under CC BY-SA 3.0 via Commons.

The RAF's Hawker Siddeley Nimrod does.

Source: Wikimedia
But only because it is an anti-submarine aircraft; its radar is designed to point at the sea, not the air. Of course you could still put weather radar in a downward-pointing nose, but that would be a waste of space and engineering. Commercial aircraft strive for efficiency, and in this case it makes more sense in terms of balanced aerodynamics and instrument placement for a jetliner to have a nose that curves both upwards and downwards.
Also take a look at another view:

Source: airliners.net
Where would you put the baggage?

Well, Nimrod is a weird example because it had a second fuselage basically strapped to the bottom of a Comet. However, Comet has a similar if less extreme version.

There are more common aircrafts. Have a look at Embraer, Avro RJ, and all that business jets. Most of them have the nose down.

Just to add to other answers, evenly sloped fuselage on the top and bottom in the front of the aircraft would create lift and drag somewhat equally in top and bottom direction, thus rendering fuselage aerodynamically neutral like BillOer mentioned in comments.
If you were to design fuselage like the vehicle below, downforce generated by the upper part of the fuselage would push the whole assembly downwards - this is desirable for road vehicles as the more planted on the road they are, the better. For aircrafts however, balance is the key:


text:
What are the specifications of this Zenith bush airplane's gear and tires?

I would like to know the name (if any) and the specifications of Zenith's bush airplane? I would like to know the size, the diameter, the maximum load, bearing number/type, and so on. Seems that the front gear and the main are different in spec.

It's a Zenith STOL CH 701. See also here and the links on that page. Also here for the gear.

The two links are good. But unfortunately I can not see there the gear specs.


text:
How do I calculate the air pressure on the maximum bulge of the airfoil?

Picture 2: source.
Consider I have an airplane and it wing, of course. The cross-section of the airfoil is as the picture 1. The leading edge and the wind direction are depicted there. The leading edge I depicted as simple as possible to make it easy to be understood. It is exactly a half of a circle. For clearer, you may see the second picture or watch the video of the link I provided above.
Then my question is, how do I calculate the air pressure in the leading edge's top bulge as in the first picture?

What is the final result you would like to get? Lift and drag? Or you need the actual pressure estimate?

Hey that's exactly what MY head looks like when I blow dry it!

Quite strange no answer this question.


text:
When Airplane is reducing altitude from flight level (cruise), which one is more efficient
When an airplane is reducing altitude, say due to preparation for landing, the nose is tilted down. That I got from a video on Youtube which explains the way an airplane is going down until it reaches a certain altitude and until the nose is revert to tilt up. Then my question:

Is that explanation true that during reducing altitude the nose is tilted down?
Which one is more efficient compare to if the airplane is tilted up as the position of an airplane when it landing. "Efficient" here is about which one faster, more fuel efficiency.
What is the allowed maximum angle tilt down nose?

You should only ask one question here. Could you please edit?

There are (mostly) two ways to reduce altitude:

Reduce speed until your wings don't generate enough lift in level flight. Typically the nose will drop by itself since you are no longer at your trimmed airspeed. You might be able to re-trim so that you are in a level descent.
Drop the nose manually without reducing (or even increasing) speed

Airliners at altitude operate very close to what is called the coffin corner where an increase in speed means you overspeed the aircraft, and a reduction in speed means you stall. So preferably the pilot will simultaneously reduce the throttles while lowering the nose to maintain a specific speed.
Tilting the aircraft up (nose up) will reduce your speed and eventually you will fall to the earth (as you stall). This is a very inefficient way to get down (not to mention it scares the hell out of the passengers). The aircraft buffets, a wing may drop if the plane is not kept coordinated, and is overall a very dangerous thing to do (see the coffin corner link above).
The reason the nose is raised while landing is so that the aircraft can bleed off extra speed and reduce the descent rate for touch down (as well as reducing the stress on the nose gear). This is also a high-drag situation and is, in that effect, inefficient.

What is the allowed maximum angle tilt down nose?

90° because after that you're tilting up...
In all seriousness you can nose-down until you hit the maximum airframe speed without causing damage. For airliners this is usually a lot more than passengers are comfortable with (especially if the stewards have the rolling carts out), so pilots make a concerted effort to keep the floor as level as possible. For smaller aircraft like the 172, you can configure it such that you won't exceed this speed and point the thing almost straight down, it's actually pretty fun.

"The reason the nose is raised while landing is so that the aircraft can bleed off extra speed and reduce the descent rate for touch down (as well as reducing the stress on the nose gear). This is also a high-drag situation and is, in that effect, inefficient."  This statement is incomplete and misleading.  The primary reason that a plane noses up prior to touchdown is to increase the wing's angle of attack, increasing lift so that the plane can continue flying at slower airspeeds (which decrease lift) prior to landing.

@SkipMiller I assumed they would understand that "reducing the descent rate" means you have to add lift. As the speed reduces further, more "lift" needs to be added so that you don't increase your descent rate, so you pull back more. Eventually you stall (in a full-stall landing) or touch down.

@RonBeyer, very nice explanation. But there is confusion for me since this statement: "90° because after that you're tilting up...", which I believe it is the answer for my third question: "What is the allowed maximum angle tilt down nose?". Is that mean 90 degree tilted down (which is mean, vertical)?

@AirCraftLover Yes, you can point it straight down, unless the aircraft has some kind of artificial limiting (would depend on a specific aircraft) then the only limit is structural damage speed, nothing else limits it.

Thank you for the nice explanation.


text:
What is the Angle of Attack of this Propeller?

Photo 1 and photo 2 from here, photo 3 is from here.
Here are three propellers. As in my understanding, the AoA or the pitch of a propeller should be close to its rotation's direction, around 15 degree on the root and less on the tip, more or less like what shown by picture 3. But this propeller with 6 blades (second picture) seems the AoA or pitch is not as that 'rule'. The AoA is close to the axis rather than to its rotation's direction.
Then my question is, how such propeller works?

I can see the tips. The way you are using the terminology is still incorrect. xxavier gave you the only possible correct answer.

So, what is the best terminology?

Let us continue this discussion in chat.

In the second photo is just a "feathered propeller" (flag position or rest position on ground to not be spinning around if the weather is windy). This pitch is not used in flight unless the engine is stopped (not working), in which case the feathered position is required to minimize drag on the aircraft from the propeller.

to be noted that pitch and AoA are related but not the same thing.

Very interesting. So, can I say that when that propeller is in use (working), the angle of attack or the pitch will be also like picture 3? I added another picture so it clearer. The third picture is fix pitch propeller.

Yes, for the fixed pitch propeller the  RPM's  adjusting, will give the thrust you need for the take of /climb regime's.

One can't speak about the 'angle of attack' of the blades of an stopped propeller in no-wind conditions, simply because the angle of attack is defined with reference to the relative wind direction, and when that reference doesn't exist, there is no angle of attack to speak of, either...

Just like my question to the above, so, can I say that when that propeller is in use (working), the angle of attack or the pitch will be also like picture 3?

While absolutely, technically correct, for the average layman, that's a pedantic detail that doesn't answer the question. The fact that the prop in the 2nd picture is feathered as noted by GeorgeGeo is much more useful to the average layman in explaining why/how that particular prop "works". i.e., it isn't in a working position in that picture.

@AirCraftLover to an extent, yes. The AoA will vary depending on the power needs, the engine RPM and a variety of other factors. As I understand it, most variable pitch props will automatically adjust themselves to suit the conditions, but that's the subject for another question - one that I'm pretty sure has already been asked here

Thank you @FreeMan. I understand what is variable pitch propeller. I was just confused to see the pitch, was not thinking about the reason.


text:
What is the difference between an aviation training device (ATD) and a flight training device (FTD)?
According to FAR 61.57, the regulation requires different experience requirements when a pilot uses FTD or ATD for their experience minimums. So I have tried to look up the difference between those two but I don't really understand.
According to Advisory Circular 61-136A, the definition of ATD is a training device, other than a FFS or FTD, that includes

a replica of aircraft instruments, equipment, panels, and controls in an open flight deck area or an enclosed aircraft cockpit.

The definition of FTD, according to FAR Part 1, is also

a replica of aircraft instruments, equipment, panels, and controls in an open flight deck area or an enclosed aircraft cockpit

They look pretty much same to me.
Could you give me the examples of those two?

This might help: https://bruceair.wordpress.com/tag/aviation-training-device/

14 CFR Part 141, Chapter 41 Flight simulators, flight training devices, and training aids. Requirements for a FTD:

(1) Be a full-size replica of instruments, equipment panels, and
  controls of an aircraft, or set of aircraft, in an open flight deck
  area or in an enclosed cockpit, including the hardware and software
  for the systems installed that is necessary to simulate the aircraft
  in ground and flight operations;

The important mention is it must be a full-size replica of controls and panels. This doesn't apply for an ATD (see definition in AC 61-136A, appendix 2).
See also:

Sorting the simulators.
FAA withdraws ATD rule.

An AATD is an Advanced Aircraft Training Device. It is the lowest qualified device by the FAA (think Redbird.) An AATD will have to simulate a cockpit of a specific aircraft. An FTD or Flight Training Device, will feature a full cockpit of the aircraft, reinforcing the proper muscle memory for training (think of a full motion sim - minus the motion.) FTD's are qualified from level 1 to level 6. Level 6 will incorporate the fight physics and aerodynamic data of the real aircraft. There is a level 7 however, it is designated for rotorcraft only. Lastly, there are FFS's or Full Flight Systems. There are only level C and D's. Only aircraft that are more than 12,500 lbs are required to be trained in a FFS and most are for acquiring a FAA Type Rating.

It would be helpful if you could link to an authoritative source document that delineates these distinctions.


text:
How can we recover from a tailplane stall?
I was studying for icing and a tailplane stall. I have looked up some internet pages and instrument flying handbook, and found the procedure below.

raise flaps to the previous setting. (To reduce down wash from the main wing so that reducing negative angle of attack of the tail and break the stall)
apply nose up elevator pressure (I don't get it. The nose up pressure will make the elevator to go up and wouldn't this increase the negative angle of attack and worsen the stall?)
do not increase airspeed unless it is necessary to avoid a wing stall. (Why shouldn't we increase airspeed?)

So now I'm trying to understand the reason why should a pilot do such actions. Can you help me out?

Please quote where you answered your own question with 1) 2) 3).

Oops, I did not mean to break this into two comments.  Define "tail stall", I assume meaning that the elevator stalls or that the canard stalls.  And in this case the recovery is automatic as the craft gains speed in some dive.

Recovery procedures are going to be different depending on the aircraft, can you clarify where you got the procedures you quoted and what type of aircraft you are asking about? Typically tail stalls are only found in ice contaminated tail surfaces. By the way, not all airfoils need a positive AOA to operate, as long as the AOA doesn't exceed where you get flow separation, positive or negative, the airfoil can produce positive or negative lift (down-force). Tails are usually designed to produce down-force at least equally as lift.

Oh I'm a student pilot studying for instrument rating. And got this procedures in instrument flying handbook and the internet while I was studying for tailplane icing. So if you are asking the type of aircraft, I would say in general, or PA 28.

Maybe I should have said "tailplane stall". I thought a tail stall and a tailpalne stall are the same thing. Sorry about that

Lets start with the very basic concepts....
In most aircraft, the Centre of Gravity (cg) is somewhat forward of the wing or mainplane Centre of Pressure. The exact distance between the cg and the Centre of pressure will depend on aircraft loading, configuration, thrust setting and drag. However, cg forward of the Centre of Pressure produces a nose-down pitching moment. The horizontal stabilizer, or tailplane, then provides a downward force to overcome this normal, nose-down, pitching moment.
The tailplane behaves as an ‘upside down’ wing and operates with negative Angle of Attack (AOA) as shown in Figure 1
Positive and Negative Angle of Attack

Figure 1 - Positive and Negative Angle of Attack
If the horizontal stabiliser becomes contaminated with ice, airflow separation from the surface can prevent it from providing sufficient downward force or negative lift to balance the aircraft and a nose-down pitch upset can occur.
When compared to an aircraft's mainplane, the horizontal stabiliser normally has a thinner aerofoil with a sharper leading edge. Differences in the ice collection efficiency or catch rate between the two surfaces means ice accumulates faster on the horizontal stabiliser and may form before any ice is present on the aircraft's mainplane.
Tailplane stall can occur at relatively high speeds, well above the normal 1G stall speed of the mainplane. Typically, tailplane stall induced by icing is most likely to occur near the flap limit speed when the flaps are extended to the landing position, especially when extension is combined with a nose down pitching manoeuvre, airspeed change, power change or flight through turbulence. Aircraft stall warning systems provide warnings based on an uncontaminated mainplane stall so during a tailplane stall induced upset there will be NO artificial stall warning indications, such as a stick shaker, warning horn or the mainplane or flap buffeting normally associated with a mainplane stall.
Tailplane Stall Aerodynamics

The horizontal stabiliser, or tailplane, of an aircraft is an aerofoil that provides a downward force to overcome the aircraft's normal nose-down pitching moment. The further forward the Centre of Gravity is from the Center of Pressure, the greater the nose down moment and, thus, the greater the amount of down-force that must be generated by the tailplane. This, in turn, requires a greater negative tailplane angle of attack (AOA). angle of attack (AOA).
[As shown in Figure 1, The tailplane is effectively an upside down aerofoil so an increase in negative tailplane AOA occurs with UP elevator movement or when the aircraft is pitching nose down.]
Accumulation of ice on the tailplane will result in disruption of the normal airflow around that surface and will reduce the critical (or stalling) negative AOA of the horizontal stabiliser.
Ice can accumulate on the tailplane before it begins to accumulate on the mainplane or other parts of the aircraft.
Flaps extension usually moves the mainplane Centre of Pressure aft, lengthening the arm between the Centre of Pressure and the cg and increasing the mainplane nose down moment. More down force is required from the tailplane to counter this moment, necessitating a higher negative tailplane AOA.
Flap extension, especially near the maximum extension speed, increases the negative tailplane AOA due to the increase in downwash, as shown in Figure 2
Increasing the power setting on a propeller driven aircraft may, depending on aircraft configuration and flap settings, increase the downwash and negative tailplane AOA.
When the critical negative AOA of the horizontal stabiliser is exceeded causing it to stall.
Tailplane stall drastically reduces the downward force it produces, creating a rapid aircraft nose-down pitching moment.

Effect of mainplane flap on downwash

Figure 2 - Effect of mainplane flap on downwash
On aircraft with reversible (unpowered) elevator, tailplane airflow changes caused by ice accretion may lead to an aerodynamic overbalance driving the elevator trailing edge down and pitching the aircraft nose down. This can occur separately from or in combination with the nose down pitching moment caused by tailplane stall. The yoke may be snatched forward out of the pilot’s hands and the control force required for the pilot to return the elevator to neutral or to a nose-up deflection can be significant and potentially greater than the pilot can exert.
now match with your recovery actions

You have no doubt with your 1st point.
The second point: You have to resist the nose down elevator movement. Once the tailplane is already stalled then you have to assume the elevator has already gone in down position to make your ac nose down. So you have to apply nose up elevator pressure.
You should not increase the airspeed cause it might make the situation worse. Cause in dive with increased airspeed is always difficult to maintain the aircraft control. And you might end up with overstressing the elevator which is not good at all in such condition.

I think a reason to not increase the airspeed is that it also increases the nose down moment generated by the wing, demanding a higher corrective force from the tail plane.

There was a NASA report on the NASA/FAA Tailplane Icing Program Overview, which covers the points raised by you. It lists certain actions that can be done to recover from a tail plane stall:

When the full tail stall was experienced during the power transition, the stall recovery procedure was:
• Reduce thrust (may be airplane specific)
• Pull back on yoke/ increase $\alpha$
• Raise flaps

Raise flaps to previous setting
This is done mainly to undo the changes that caused the stall in the first place (reducing thrust is also done for this reason). The report says,

The major lesson learned to recover from a tail stall was to undo what was
just done to cause the event.

Apply nose up elevator pressure
Basically you are pulling back on the yoke to increase the tail download so that the aircraft nose-down pitching moment is countered. From the report:

Pulling back on the yoke increased the camber of the tailplane, which provided enough tail download to counteract the nose-down pitching moment and increase the $\alpha_{tail}$

This sounds counterintutive as the conventional tail produces force in the direction opposite to the main wing. The report notes:

It was noted that this tail stall recovery procedure is opposite of the recovery from a wing stall. The reason for the difference is the location of the flow separation. In a wing stall, the flow separates from the upper surface of the wing, therefore reattachment is made by decreasing the wing $\alpha$. In a tail stall event, the flow separates from the lower surface of the tail and requires a positive increase in tail to reattach the flow.

Do not increase airspeed unless it is necessary to avoid a wing stall
The aircraft is already at a nose down attitude. Increasing speed will further excabarate the situation, which may put the corrective action beyond the capability of the tailplane.

I have a theory on applying the backstick on a tail stall induced while lowering the flaps. OP’s comment:

apply nose up elevator pressure (I don't get it. The nose up pressure will make the elevator to go up and wouldn't this increase the negative angle of attack and worsen the stall?)

If the tail stalls, then the nose pitches forward. The center of rotation is about somewhere near the wing and CG, well forward of the tail. So the tail instantly sees a step increase in AOA, to well beyond the stall AOA. Probably really high AOA. The aircraft develops inertia in pitch too.
At this point, you are doing triage while you lower the flaps and undo what you did. While you’re not getting much lift (downward) from the tail, it is resisting the forward pitch with drag. Letting the stick or yoke come forward would dump this pitch resisting force, and the aircraft would go right over its nose before the flaps could raise.
There’s my theory.

For those recommended actions to be effective, two preconditions have been quietly assumed:

The tail surface produces downward lift and
The wing has positive camber.

Both can be assumed to be correct in almost any case. Now let’s look at the three recommendations in detail:

raise flaps to the previous setting.

Flaps increase camber and shift the center of pressure backwards. In order to balance the aircraft with the same center of gravity location, the tail needs to produce more downward force with lowered flaps. Retracting flaps will unload the horizontal tail and reduce the stall condition.

apply nose up elevator pressure.

This adds tail camber and helps to produce the same downward force at a less negative local angle of attack of the stabilizer. The induced angle of attack of the added tail camber will increase the local angle of attack at the stabilizer. This can only help momentarily, though, because it will make the aircraft pitch up and lose speed - unless you have a movable stabilizer which is used for trim. Re-trim with the new elevator setting and the change becomes permanent.

do not increase airspeed unless it is necessary to avoid a wing stall.

When the main wing has positive camber, a lower wing angle of attack shifts its center of pressure backwards. Therefore, the tail load and lift coefficient are lowest at low speed, and flying slowly will unload the tail. With rear center of gravity, tail load normally can even become slightly positive at low speed.
So in all cases the recommendations help to unload the tail and reduce the condition that let the iced tail stall.

The “do not increase airspeed” is interesting. The pitch angle of the tail, as measured by yoke position, programs forward as speed increases. I’ve measured this repeatedly. Therefore, the angle of attack decreases. Yes, the center of pressure of the wing moves aft, but the moment from the tail dominates. So I’d have voted for going faster to help with tail stall.

@MikeY … which is wrong. Yoke position only tells you about elevator angle, not pitch angle. At low speed the angle of attack on the tail is higher and the lift coefficient smaller (or even positive). When you go faster, the wing moment grows with dynamic pressure and the rear shift of the center of pressure, and both will demand higher downforce and more negative lift coefficients from the tail.

What do you mean by “lift coefficient smaller”? You also just said is operating at a higher AOA. Those two statements conflict.

Also, Piper Warriors have all moving tails, so I wonder if yoke aft is the best recovery procedure for them. Of course with a Warrior in icing...probably going down anyway.

@MikeY Of course is a higher angle of attack coupled to a smaller lift coefficient - we are talking about a downforce here.

authors: Виталий Боков


text:
What is this object name and how it works?

Photo is from here.
I believe that it something like steer on a car. But seems that it has function or control more than just turning left or right. Is any different between left and right?

Does it honk when you press it?

@Jimmy Indeed, some aircraft have horns!

Sorry for being a little blunt, but do you have absolutely no knowledge of aircraft? It's the most basic control input device, the yoke. It controls the three main axis of movements, the pitch, yaw and roll.

It is a yoke.  Turning it left or right rolls the airplane.  Pulling it towards you pitches up.  Pushing it away from you pitches down.

How about turning the nose wheel when it preparing for take off and when it going to park (what is the suitable name for this)? Or, is turning the nose wheel during preparation for take off also using aileron?

The nose wheel is usually steered with the rudder pedals.  Sometimes (especially with tail wheel airplanes) the "steering" tire just casters, so when taxiing you steer by braking the left and right main wheels separately.  Some larger aircraft use a separate "tiller" for steering when taxiing.

Unless you are flying an Ercoupe, I think that's the only aircraft where turning the yoke (wheel in an Ercoupe) turns the nose wheel.

@CamilleGoudeseune, will be still effective to control the nose wheel with the very low speed like during preparation for take of or "park" which the speed sometimes something near 0 km/h? I doubt really works due the power hitting the rudder is not sufficient.

@RonBeyer, are you saying that only Ercoupe that nose wheel is controlled with rudder?

Ercoupe steered the nosewheel with yoke because it had no rudder pedals.  @aircraftlover study this carefully: https://www.si.edu/object/nasm_A19790677000.

@aircraftlover Nose wheel is controlled with rudder PEDALS, not with rudder.  Spend a few weeks with textbooks from your library or with someone friendly at your local airport.  That will teach you the basics  more gently and quickly than getting hints one at a time over many months from an online Q&A site.

@CamilleGoudeseune, firstly, sorry for the mistake about rudder and rudder pedal. Second, I have accepted. Third, what I need to study from the link above? It is about ERCO Ercoupe 415-C. The cockpit above is Cessna 152's.

@CamilleGoudeseune, please tell me how the yoke linked to the rudder pedals so it can be functioned as steering wheel? That will solve the question in this post.

The Ercoupe link explains why it's unusual, why it had no independent control of rudders, and why nobody copied this design that was intended to make car drivers more comfortable in an airplane.
The Ercoupe's yoke controlled ailerons AND rudder AND nosewheel.
No other airplane's yoke controls the nosewheel.
The C152 is utterly traditional in that the yoke has nothing to do with the rudder or the nosewheel.

@AirCraftLover The yoke in most aircraft is not at all linked to the rudder pedals. Steering on the ground is done completely with your feet. You push on the left pedal to go left, push on the right pedal to go right. It is linked through a system of pulleys/bungies to the steerable nose wheel.

@CamilleGoudeseune thank you for your explanation.

@RonBeyer, nice info. That help me understand how an airplane controlled when in ground.


text:
Which type of fan is better for wind tunnel testing?

I would like to build a small wind tunnel to test a miniaturized wing/airplane. I have two options, using axial blower (picture 1) or using centrifugal blower (picture 2). The first will be the wind source for "pipe" tunnel while the second one will be connected to a ducting tunnel. The size and the power (cfm) will be provided as per requirement.
Then my question: which one is better of the two? Or, is any other better alternative?

Purely anecdotally, all of the videos of aircraft wind tunnels I've seen use a circular fan like the first image, and not a blower like the second image.

What is that mean Purely anecdotally,... Sorry for my poor English.

No worries! It means I don't have an official source, I've just noticed it myself.

Thank you anyway. At least I have one reference.

You could make either one work, especially for a "straight  through" tunnel.  The key is to smooth out the turbulence after the fans to as straight and undisturbed a line as possible.
Smoke trails help check your wind stream before the model is put into place for testing,
and remain valuable to see changes in the wind flow pattern as it passes over the forms.
There is an excellent article on the internet:  howstuffworks wind tunnel, and a very good diagram showing post fan stages such as stators, vanes, diffusers, and settling chamber that help create a desirable flow.
For testing of lower subsonic designs, a straight through tunnel, such as the one the Wright brothers built, may be easiest to construct.

If we use smoke, what will be the density?

I will check the howstuffworks wind tunnel.

A small amount of smoke will not greatly affect density.  Several lines between clear air are best.


text:
How many ways/methods to calculate propeller/rotor's power output? Which one is the best?

Source: Wikipedia
I want to know how many ways/methods to calculate the output power of a propeller? So far I knew three methods:

Momentum Theory (MT),
Blade Element Theory (BET),
Blade Element Momentum Theory (BEMT).

My question are:

Is any other way/method?
If any, what way/method is it?
And what is the best way/method to calculate the output power of a propeller (Power = Thrust x Velocity)?
Are those ways/methods applicable for Computational Fluid Dynamic (CFD)?
If those ways/methods are applicable for CFD, which one is the best?

There are many approaches that have been used to model propellers.  Each has their pros/cons and tradeoffs of what they can capture vs. their computational cost.
Momentum theory (also called actuator disk theory) treats the prop as an impulsive pressure jump -- there are no individual blades modeled.  It does not capture the detailed shape (chord, twist, lift coefficient) of the prop.  It usually does not model a complex load distribution -- but can be extended to load distributions.
Blade element is basically a strip-wise integration along a blade.  Each strip is treated as if it is in 2D flow.  There are some empirical tip-loss factors that can be applied.  This theory does not generally capture the induced inflow to the propeller well.  There is no blade-to-blade interaction in this model.
BEMT combines the above to better capture the induced inflow to a propeller.  It still does not capture blade-to-blade interaction.  It still uses an empirical factor for any 3D effects.
Next you go to potential flow and vortex theories that can start to capture discrete blade effects and blade-to-blade interaction.  These can also better capture 3D effects.
Then you move to more traditionally numerical methods.  Rotating lifting line and rotating vortex lattice -- these can be done in an unsteady formulation, or in a steady formulation with a rotating reference frame.
Then you move to a rotating panel code, followed by various CFD formulations.
The ability of these methods to predict thrust and power varies on the flow condition.
A heavily loaded prop (high thrust and/or static/hover conditions) is harder to predict than a lightly loaded cruising prop.
Tips that are transonic or supersonic complicate matters.
Heavily loaded props that exhibit significant blade-to-blade interaction and 3D effects may have 3D shapes that can not be modeled by some of the more simple methods.

Thank you for your explanation. I just didn't know where to go. Normally in Wikipedia will be referred to another link if any, but I didn't see it there. Let me dig deeper to those methods you mentioned. Thanks a lot.

Btw, how accurate do you think the Momentum Theory aka Actuator Disk Theory if I used to calculate subsonic angular speed propeller?

Actuator disk theory is extremely limited.  It only calculates the ideal induced power for a uniformly loaded disk.  It can not calculate viscous and other sources of power.  Thrust is an input to actuator disk, so it captures that exactly.  Actuator disk theory is only based on conservation of mass and conservation of linear momentum -- it can't be very wrong.  It just might not give all the answers you want.

From those three methods I mentioned above and those another methods you mentioned, which one is the (still) acceptable method to calculate propeller output power for subsonic angular speed (common propeller, turbo fan is not included) used by hobbyist? Means, maybe the result is not fully accurate but still acceptable, yet not to complicated.

They are all acceptable under certain circumstances.  There is no easy answer here.  I would suggest you first look at the UIUC propeller database https://m-selig.ae.illinois.edu/props/propDB.html, the data provided by APC Propellers https://www.apcprop.com/technical-information/performance-data/ or if you really want to do computations, XRotor https://web.mit.edu/drela/Public/web/xrotor/, QProp https://web.mit.edu/drela/Public/web/qprop/, or CCBlade https://github.com/byuflowlab/CCBlade.jl or VSPAERO http://www.openvsp.org


text:
How does the V-280 Valor maintains its Center of Gravity (CG) when it is converted from helicopter (hover) mode to plane (flight) mode?

Picture source:
The above pictures are showing three modes of V-280 Valor: hover or VTOL/hover mode, transition mode from VTOL to flight, and flight/air plane mode. As we may see, there is a CG shifting in that conversion, its CG must be shifted backward to compensate the weight of the two rotors and their supporting cases. I guessed its CG is exactly at the rotors' interconnecting shaft. But when the rotors are tilted forward, the CG must be shifted backward as the load-factor of the rotors/propellers are changed significantly. Indeed, during the tilting forward, the front side load maybe supported by the thrust generated by the rotors/propellers. But after they are inline to the fuselage, no more thrust can compensate. As we know, the tilt rotor of V-280 Valor is different than tilt rotor of V-22 Osprey, which with the V-22, the engines themselves also involved, they are tilted backward-up so the CG remains in its location.
My question then, how does the V-280 Valor maintains its Center of Gravity (CG) when it is converted from helicopter (hover) mode to plane (flight) mode?

On most aircraft, as fuel is burned, the position of the CG changes, albeit slowly, and the pilots or autopilot compensate for that.  Given that the transition between hover and forward flight is not instantaneous, why do you believe that the CG has to "be maintained", rather than it shifts and that shift is compensated for with the flight controls?

@RalphJ, in most aircraft, the fuel tank is located on the CG. Indeed, it is changed time after time, but the changes is very slowly. And the fuel flow is designed such that way so will now flow so fast when the aircraft being banking such as when making turn. Control the fuel flow is the way to handle it. So, what is the way to handle its CG shifting during the transition in this V-280 which it is changed quite rapidly compared to that fuel changes?

First an foremost, the CG does shift, I strongly believe that there is not counter-action to this. Furthermore I speculate that this is a clever design feature in order to satisfy two needs in flight (however I cannot proof this...)

In Hover flight, the CG is exactly under the rotor shaft, therefore no cyclic trim input is needed which reduces the stress on the shaft and also on the actuators as the aircraft can hover without the need for constant cyclic input.
In forward flight, the CG shifts forward, as you already noticed in your question. This has the added benefit that the forward CG adds more longitudinal stability in forward flight. Infact this is needed, as the V-280 would otherwise be unstable in forward flight, which would vastly complicate the flight control system.

This CG-shift is probably already the case for the V-22, it is just not that obvious as the entire nacelles rotate.
Unfortunately I could not find a good reference quickly, but you might be interested in this dissertation, on page 55 the trim values for a V-22 like aircraft is given.

Quote: "Furthermore I speculate that this is a clever design feature in order to satisfy two needs in flight." Yes, indeed, a very clever designed, of course.

"In Hover flight, the CG is exactly under the rotor shaft, therefore no cyclic trim input is needed which reduces the stress on the shaft and also on the actuators as the aircraft can hover without the need for constant cyclic input." Of course it should not use cyclic to overcome the CG shifting. It will waste power a lot.

"In forward flight, the CG shifts forward, as you already noticed in your question. This has the added benefit that the forward CG adds more longitudinal stability in forward flight." That is part of my question and it is why I ask this question as every plane has slight pitch down  as it is required to maintain the airplane glide in case total power failure.

@AirCraftLover: in helicopter mode the CG must not be exactly under the rotors (which is impossible) rather it must be within a well defined forward and backward limit, limit that normally depends on the weight.

@AirCraftLover given that the V280 features a fully digital flight control system, the V280 will most definitly crash given a total power failure. Loss of power would mean no more control, as the chain from pilot input to actuators would be cut. In that case the aircraft would enter some sort of dive/spin/uncontrollable attitude or all of these options, therefore inevitably crashing

Perhaps I should clarify, I mean in the event of total electric power failure

The center of gravity does not need to be exactly under the rotor shaft in helicopters or tiltrotors. This would be difficult to manage, and to mitigate the problems involving non aligned CoG and lift, helicopters and V-22 Osprey, as well as most likely the V-280 Valor use swash plates that transfer the center of lift of the rotor disk(s) as per commands from the pilot or flight computer.
During transition, before the forward speed is sufficient to create balancing force from the tail assembly, as the rotor is tilted forward the balance of CoL vs CoG is maintained by shifting the CoL as necessary via the swash plate.
Also note that during transition the lift must be increased in the beginning stages if level flight is to be maintained. As the rotor tilts forward, the vertical component of the lift decreases, and it must be increased before forward speed is sufficient for the main wing to produce lift. This is also done by the swash plate by incresing AoA of the blades.
There is speculation in the web about V-280 not using a swash plate, but instead conteolling the AoA of each blade individually by electric motors, but I was unable to find any authorative source for this.

"The center of gravity does not need to be exactly under the rotor shaft in helicopters or tiltrotors. This would be difficult to manage, and to mitigate the problems involving non aligned CoG and lift, helicopters and V-22 Osprey, as well as most likely the V-280 Valor use swash plates that transfer the center of lift of the rotor disk(s) as per commands from the pilot or flight computer." If the CoG is not below the interconnecting shaft, then the cyclic must be applied, and it will reduce lifting force several percent. But however, it is part of my question. Is it the case?

"During transition, before the forward speed is sufficient to create balancing force from the tail assembly, as the rotor is tilted forward the balance of CoL vs CoG is maintained by shifting the CoL as necessary via the swash plate." For short period of time, it is possible to apply cyclic to the aircraft, but not for long time as the efficiency will be reduced significantly depend of the AoA is applied. If it is negative, the power reduction will be > 25%. But then, what about during the flight?

In normal  airplane, CoG is maintained by the horizontal stabilizer. But in this V-280, is that technique also applied? Indeed, as the V-280 also an airplane, the airplane must be slightly tilted down to make it glide able if the case all power failure. But however, it is just my opinion, I am asking the firm answer.

Yes, cyclic must be used during hover and transition, as I described in my answer, to maintain balance. During flight in airplane mode, it is not needed.

Why wouldn't the V-280be able to tilt downwards to glide? Of course it can. During airplane mode it is contolled as any other airplane is.

@Jpe61 i am not sure how well the v280 or any other of these convertiplanes glide... That is actually a really interesting question imo

My guess is they glide better than the space shuttle, which in turn glides better than a brick

how does the V-280 Valor maintains its Center of Gravity (CG) when it is converted from helicopter (hover) mode to plane (flight) mode?

It doesn't. And there's no need for the CG to be maintained fixed among helicopter and aeroplane configuration.

In helicopter configuration, CG should be as much as possible centered between and under the two rotors in order to minimise stress on the rotor shafts and this limitation is for sure valid also for the V-22 and the V-280. The limits within which the CG must lie is called CG envelope and depends mainly on the weight, being the envelope smaller at higher weight. For example for the R44 it looks something like that (source; on x-axis distance from the nose; on y-axis weight):

In transition phase the two rotors have to be rotated forward just of a very small angle to gain forward speed, with a relevantly small forward shift of CG as well. Just as an example, main rotors of conventional helicopters are tilted forward of some 3 to 5° and that's enough to make them reach forward speeds as high as 300kmh (200mph). This small forward CG shift has anyway a smaller impact than if it was toward the back, as can be seen from the previous plot.

Finally, in aeroplane configuration CG shifts are less of a problem than in helicopter configuration since CG shifts are compensated by the horizontal stabiliser, just like any conventional aeroplane. And just like any conventional aeroplane, CG must lie within the limits of the longitudinal stability, as well explained for example in this answer.

By the way, since combustion engines are quite heavy (as much as some 10% of the empty weight) and since in the V-22 they rotate too during transition phase, I would actually expect a bigger CG shift in the V-22 compared to the V-280, where only the two relatively light rotors rotate.

Quote: "Anyway in aeroplane configuration CG shifts are less of a problem than in helicopter configuration since CG shifts are compensated by the horizontal stabiliser, as long as the limits of the longitudinal stability are respected." Yes, indeed, but still need to be compensated. Every changes must be compensated. But my question, how does it compensated/maintained? I am asking the way it does.

How longitudinal stability works is another lengthy topic, you can find a good answer about it here. I linked that answer also in my text.


text:
Why do I never see fighter jets on sites like FlighRadar24 and ADBSExchange.com
On ADS-B Exchange they even have a filter option for military aircraft, and it will show all kinds of transport, mid-air refuelers, helicopters, and even training aircraft like the Texan, but I don't think I ever see fighter jets on there. Even when I see some F/A-18s fly overhead IRL I still never see them show up on any websites. Or like right now I noticed that Air Force 1 was flying out of Palm Beach I think it was but I didn't see any fighter escorts show up. Are fighters allowed to just turn off their transponders in civilian airspace?

Does this answer your question? Do flight tracking websites also track non-commercial and defense aircraft?

Because they don't want to be seen.

Are fighters allowed to just turn off their transponders in civilian airspace?

FR24 and ADS-B Ex are not based on normal transponders (that would require access to actual RADAR data) but rather ADS-B and mode-S transponders. The military can simply choose not to operate with this equipment. This does not make the fighters invisible on normal ATC radars, since ATC obviously still needs to be able to see them.


text:
What would cause a non-zero indicated airspeed reading on the ground?
What kind of failure would cause an indicated airspeed of say 200 knots on the ground (stationary)?

Have you ruled out a hurricane or tornado?

@StephenS Can definitely confirm the airspeed was actual airspeed was fine haha.

I was about to question your intelligence thank goodness you ruled out heavy winds.

A mechanical blockage inside the airspeed gauge could prevent the needle from dropping all the way back to zero -- assuming the aircraft had exceeded 200 KIAS in the first place.
Whether this or the pitot-related failures in the @CamilleGoudeseune answer, the pitot-static unit, airspeed indicator, and altimeter (also connected to the static port) should be checked and repaired as needed by a qualified person.

This would happen if you were flying at 200 knots, encountered enough freezing moisture to completely ice over the pitot tube and the static ports, didn't have pitot heat for some reason, and then (somehow safely) landed and parked, while still iced over.
Once iced over, IAS won't change.

Any other source of clogging the pitot tube with pressure inside would do the same (give or take the IAS).  Even putting a protective cover on it, if that produced an airtight seal, could cause the airspeed to read while the cover was in place.

Yes, although any cover for sale that even mentions airtightness say that it's not airtight!  Homemade, on the other hand...

@CamilleGoudeseune If the static port were completely iced over, I'd expect to see  the altimeter not functioning properly, but it is. So at that point, and I guess this is what the other commenter was referring to, just the obstructed pitot tube could still result in this error?

IAS comes from the difference between ram pressure and static pressure.  A blocked pitot tube freezes the former, a blocked static port the latter.  So with only the pitot blocked, a change in elevation also changes IAS.  Maybe it might still read 200 kts when parked, but that's trickier.

Descending with a blocked pitot would decrease the difference, reducing displayed IAS.  If the IAS had been reading falsely high at altitude, it might still read 200 kt parked.  Bottom line here is that a qualified person should be brought in to inspect and repair the pitot/static unit and attached instruments.

Blocked pitot tube left for a few days and a different weather system coming to cause a drop in ambient pressure could cause a differential to register in the ASI and display a speed when stationary.

How much would ambient pressure have to change to result in 200 KIAS to be shown?

@Bianfable I think that the idea here is that it was first blocked at some airspeed (say, 100 knots) and then the difference was exacerbated by a change in ambient pressure? So it would depend on at what point the pitot tube became blocked.

If it was something a little more reasonable like 20 or 50 knots, it could be from a headwind.

"If it was something [else]..." does not answer the question.


text:
For an aircraft, what are the differences between nonlinear and linear equations of motion?
So, as far as I understand, linearized equations of motion are just nonlinear equations of motion converted to linear EoM's using approximations and assumptions to simplify/remove elements from the equations.
In an aircraft, what are these nonlinear EoM's? If, for example, I stuck my airplane in a wind tunnel to determine the entire set of aerodynamic coefficients, would that cover all of the nonlinear portions of the equations of motion?
Thanks!

No, wind tunnel measures aerodynamics. EOMs capture inertial and dynamical effects on a mass. They stand apart.

Are you asking for a description of what "nonlinear equations of motion" means in the context of aircraft, or are you asking for the equations themselves?

Improve which particular bit of the fidelity of the simulator? Are there any aircraft dynamic responses matched by simulator responses?

@Koyovis, in particular, the post-stall behavior and the behavior in some high-speed, high-G maneuvers. The model generally has pretty good fidelity, we've done a relatively extensive study of flight data that matches the simulation pretty well. I'm generally okay with most aspects.

Equations of motion (EQM) define how a rigid-body accelerate and rotate given any arbitrary external forces and moments. They are no more and no less than Newton's Second Law. When expressed in body-frame, the EQMs are:
$$\bf{F}_B = m (\dot{\bf{V}}_b+\bf{\omega }_b \times \bf{V}_b)$$
$$\bf{M}_B=\bf{I}\dot{\bf{\omega }}_b$$
where $\bf{F}$ is body forces, $\bf{M}$ is body moments, $m$ is mass, $\bf{V}$ is inertial velocity, $\bf{\omega}$ is angular velocity, $\bf{I}$ is moments of inertia, and $\times$ is the cross-product operator.
The above are vector equations, which correspond to 6 scalar equations. At this point, there is no concept of aerodynamics. Notice how the equations are inherently nonlinear, so long as there are body forces/moments that depend on the state variables (e.g. velocity, orientation, etc). This is true for most forces, with the notable exception being gravity.
Aerodynamics is nonlinear in that aerodynamic forces and moments do not change linearly with flow incidence, rate of rotation and velocity, except if the changes are small. In general, quasi-steady aerodynamic force is a nonlinear function of flow incidence, rate of rotation, Mach number, Reynolds number, etc. To capture these, N-D lookup tables constructed from estimated/measured data are generally employed in simulation models.
Quasi-steady assumption itself may not be adequate in some flight conditions, such as during rapid maneuvering or stalled flight. In these circumstances, the description of aerodynamics may need to be time-variant.

It would be a nice touch to not only list the formulas but also to explain what the letters mean. Not everyone is born with that knowledge.

@PeterKämpf Amended with legend

@Koyovis where do you see force = mass x velocity?

Yep, dimensions are right.

@synchh Normal force as an input? Is it to capture aeroelastics? This would not be a usual aerodynamic parameter. Unless your model has some formulation for time, hysteresis or other time-variant behavior, I would consider it quasi-steady.

@synchh Not sure how I can help without knowing the specifics. If you've specific questions, please post separate postings.